1. Field of the Invention
The present invention relates to an antireflection film, a polarizing plate and an image display device.
2. Description of the Related Art
Antireflection film is generally disposed in an image display device such as cathode ray tube display device (CRT), plasma display panel (PDP), electroluminescence display (ELD) and liquid crystal display device (LCD) on the outermost surface of the display device where the reflectance is reduced by using the principle of optical interference so as to prevent reflection of external light which causes reduction in the contrast or reflection of an image.
Such antireflection film can be prepared by forming a low refractive index layer having a suitable thickness on the outermost surface of a support and, in some cases, a high refractive index layer, a middle refractive index layer or a hard coat layer between the support and the low refractive index layer. In order to realize a low refractive index, a material having a refractive index as low as possible is desired as a material for the low refractive index layer. Also, since the antireflection film is used on the outermost surface, it is expected to also function as a protective film for a display device. Further, the antireflection film is required to resist adhesion of stain or dust and, even when suffered adhesion, permit easy removal thereof, and have a strong scratch resistance under any storage condition.
In order to reduce refractive index of a material, there are, for example, a technique of introducing a fluorine atom-containing organic group into a binder and a technique of reducing density of the layer (introducing voids). In the case of introducing a fluorine atom-containing organic group into the binder, there arises a tendency that cohesive force of the binder itself is decreased and, in order to compensate for the decrease of cohesive force, it becomes necessary to introduce a binding group. Thus, there exist a practical limit as to reduction of refractive index, and it has been difficult to reduce it to a level of 1.40 or less. On the other hand, a technique of reducing the refractive index by introducing microvoids into the low refractive index layer enables to attain a refractive index less than 1.40. This technique, however, has such defects as that film strength of the layer is weak and that stains such as fingerprints or oil easily invade thereinto.
For example, JP-A-6-3 501 discloses low refractive index layers wherein fine pores are formed in a binder. Also, JP-A-7-48527 discloses a technique of reducing refractive index by using porous silica.
Further, JP-A-2001-233611 discloses an antireflection film containing hollow silica particles in a low refractive index layer.
However, all of the techniques described in JP-A-6-3501 and JP-A-7-48527 are not practically satisfactory in the points of film strength and resistance to fingerprint stain. Also, antireflection films using these materials have been found to have a tendency of suffering reduction in film strength under ozone exposure which simulates long-term use in a usual room.
Also, although antireflection films containing hollow silica particles in a low refractive index layer as described in JP-A-2001-233611 truly have improved scratch resistance and resistance to adhesion of stains such as fingerprints in comparison with conventional antireflection films, there have been found problems that the film is destroyed by saponification upon preparation of a polarizing plate and that, when water drops newly deposit thereon, their traces remain.